BackgroundCongenital heart disease (CHD) is typically detected during fetal life, infancy, or early childhood. However, there is no published data regarding the proportion of congenital heart defects that are diagnosed in adulthood or the impact of these defects. MethodsRetrospective analyses of all consecutive patients (N=1010) referred to an adult-CHD unit between 2018-2023. We analysed the proportion of cases diagnosed in adulthood, defining the type of defect, reasons for diagnosis, complications and need for intervention. ResultsOf total, 26.5% patients were diagnosed in adulthood (mean age 47±16 years). 75% were in NYHA class I/IV. The majority were mild complexity lesions (57.5%). Most common diagnoses were pre-tricuspid shunts, including ostium secundum atrial septal defect (ASD, 23.9%), partial anomalous pulmonary vein drainage (18.3%) and other types of ASD (5.9%). Bicuspid aortic valve (16.8%) and aortic coarctation (8.2%) were common. Other diagnoses included Ebstein's anomaly (5.6%), ventricular septal defect (4.5%), patent ductus arteriosus (2.6%) or congenitally corrected transposition of the great arteries (2.6%).The main reason for diagnostic work-up was cardiac symptoms (28.4%) such as dyspnea (19%) and palpitations (7.1%), followed by incidental findings on imaging (25.4%). A total of 47.4% had some complication, the most common being pulmonary hypertension (24.3%). Surgical repair was required in 27.2% and 25.4% underwent percutaneous intervention. ConclusionsAbout one fourth of patients with CHD were diagnosed in adulthood, up to 42.5% had moderate or severe complexity lesions. A significant proportion had developed complications at the time of diagnosis and half of them required intervention.